The present invention relates generally to structured dielectric materials exhibiting photonic bandstructure, and in particular to a new class of three-dimensional photonic layered media.
The capabilities and design of optical elements, meaning such elements are capable of controlling the propagation of electromagnetic radiation ranging from microwaves through the ultraviolet, depends in large measure on utilizing, and ideally controlling, the optical properties of materials. In traditional optics, optical elements are made of curved pieces of such materials as glass, whose curve and intrinsic optical properties determine the effect of the optical element.
A recently-developed class of optical materials is that of photonic crystals or photonic lattices. The examples produced to date have typically been composed of structured materials having periodically varying dielectric constants, the space group of the periodicity being quite simple (simple cubic, fcc, bcc, diamond, etc.) Such materials have bulk optical properties which are primarily determined by multiple scattering and interference effects due to the periodic dielectric constant.
In many ways the photonic bandstructure resulting from the periodically varying dielectric constant is analogous to the electronic bandstructure observed in crystalline solids. The motion of electrons in solids is governed by a set of momentum-energy dispersion relationships called the electronic bandstructure. Electronic bandstructure is traditionally described using such concepts as reciprocal space, Brillouin zones, dispersion relations, Bloch wave functions, and electronic bandgap, all of which appear to have counterparts in the motion of photons through periodically structured dielectric materials.
An interesting and useful optical behavior appears in periodic structured dielectric materials which are analogous to semiconductor materials. In such materials, there exist photon modes, characterized by photon energy, momentum, and polarization, which cannot propagate within the material, that is, these modes undergo self-destructive interference on interacting with the spatially periodic dielectric constant. If there is a range of photon energies for which no propagating modes exist, the material is said to exhibit a complete photonic bandgap, in analogy with the electronic bandgap in a semiconductor. Formation of a complete photonic bandgap in periodically structured dielectric materials where the periodicity appears in only one direction can occur with vanishingly small dielectric constant variation. In multiple dimensions, however, the range of variation of the dielectric constant of the medium must be rather large (usually greater than about 2) to produce a complete photonic bandgap.
Even when the dielectric constant of a medium does not vary sufficiently to result in a complete photonic bandgap, the spatial variation will often significantly alter the photon dynamics from their equivalent bulk values. Even if this occurs only over a small range of energies, this effect, and the accompanying large optical dispersion in that photon energy range, can be beneficially used in optical elements.
Applicants will use the term xe2x80x9cphotonic layered mediumxe2x80x9d to describe any dielectric structure comprising an ordered layer of structured dielectric layers in which the density of propagating photon modes or the effective refractive index of the structure varies, at some photon energy, by more than about 10% from the equivalent quantities in a homogeneous dielectric material having the same volume averaged composition. Note that although periodic photonic crystals are photonic layered media, the xe2x80x9cgeneric xe2x80x9d photonic layered medium is not a periodic photonic crystal. The intent here is that a photonic layered medium should have dielectric properties which are the result of the spatially variable dielectric constant. Thus, it is not simply a structured dielectric material, but rather is a novel construct with significant utility as discussed earlier.
Theoretical comparison of ideal and defective infinite periodic photonic crystals suggest that minor variations in structural dimensions, materials, or symmetries, including finite-size effects, cause only minor changes in the expected behavior of the ideal structure. As a result, for the purposes of this application structures which are related to a precisely described structure by processing effects, such as dimensional tolerances, interdiffusion, interfacial effects, and finite-size effects are to be considered as equivalent to the structure described.
Periodic photonic crystals have been limited to a small number of rather simple, high-symmetry materials. An early example of a three-dimensional periodic structured dielectric material consisted of a slab of optical material penetrated by a matrix of drilled holes so positioned and oriented (along the (110), (011), and (101) axes of a slab with a (100) surface) so as to produce a pair of interpenetrating connected lattices of diamond symmetry, one consisting of the slab material, and the other consisting of air. This structure is commonly known as Yabonovite, and exhibits a complete photonic bandgap when the dielectric constant of the slab material is sufficiently large.
Another class of periodic dielectric structures which can yield complete photonic bandgaps have been defined by Ho et al, U.S. Pat. No. 5,335,240, issued Aug. 2, 1994, and its continuation-in-part, Ozbey et al, U.S. Pat. No. 5,406,573, issued Apr. 11, 1995, both representing work carried out at Iowa State University. These structures consist of a periodic layered stack of dielectric rods. In each layer the rods are parallel with a constant spacing, and their orientation direction and lateral position varies between layers xe2x80x94often to result in a four-fold periodic repeat distance vertical to the layers of the stack. These are sometimes called woodpile structures. Fan et al, in U.S. Pat. No. 5,440,421, issued Aug. 8, 1995, and its continuation-in-part, Fan et al, in U.S. Pat. No. 5,600,483, issued Feb. 4, 1997, teach a related structures which also exhibits a complete photonic bandgap given sufficiently high dielectric contrast. This structure consists of a stack of structured dielectric layers, where each layer consists of a stratum of a first material having a first dielectric constant and a plurality of parallel grooves filled with a second material having a second dielectric constant. Once the stack of layers is formed, a plurality of parallel channels is etched vertically through the stack, so that the channels are perpendicular to the plane of the layers. These channels must be positioned with two-dimensional periodicity, the result being that the overall structure has three-dimensional periodicity. The cross-section of these channels can be of any shape, but their shape and size is taught as remaining constant as the channels penetrate the layers.
It is extremely difficult to fabricate such apparently simple periodic photonic crystals, particularly where the photonic bandstructure appears in the infrared or visible optical range. They also offer only a very limited range of photonic bandstructures to exploit in devices. Accordingly, it is important to develop new classes of photonic media which can be easily fabricated, preferably using ordinary semiconductor fabrication techniques. It is more favorable if the new classes of photonic media also providing for a wider range of photonic bandstructures for application. This need is at least partially addressed by the photonic layered media of the instant invention.
A new class of photonic layered media has been developed. These media can be decomposed into a stack of structured layers, which in turn consist of a collection of dielectric segments. The layered photonic media of one implementation of this new class have compact segments in the structured layers, but do not have smooth columns of dielectric passing through the medium. These new structures lend themselves naturally to fabrication using layer-by-layer growth techniques, and allow considerably more flexibility in design and detailed photonic properties than do the prior art periodic photonic crystals.